A limited number of attacks based on the base attack bonus and the normal two-weapon fighting rules are needlessly complex. A simpler approach is as follows:

Regardless of base attack or natural attacks or number of limbs, a standard action always yields one attack and a full-attack action always yields two attacks unless noted otherwise by a special ability.

You may attempt additional attacks with your full-attack beyond your normal limit up to 1 + 1/5 your level but for every attack you make beyond the normal limit it enforces an accumulative penalty to all your attacks (-6 for each additional two-handed weapon attacks, -4 for each additional one-handed weapon attacks, -2 for each additional light or natural weapon attacks). The penalty is determined by the weapon size of the additional attack.

Each Two-weapon Fighting feat or Rapid Shot feat allows you to take one additional attack before you start taking these penalties.

Creatures with special abilities or natural attacks granting them more than a normal number of attacks instead gain Two-Weapon Fighting as a racial feat. Epic creatures with these abilities also gain Improved Two-Weapon Fighting as a bonus racial feat.

This variant grants characters more flexibility in their attacks with the ability to choose a rash flurry of strikes or a few well-aimed strikes.

Examples: A 5th level Fighter with Two-Weapon Fighting and Improved Two Weapon Fighting wielding a greatsword would be able to make up to four attacks at no penalty, or make a fifth attack with the greatsword if he accepted a -6 penalty to all 5 of the attacks or a sixth attack with the sword if he accepted a -12 penalty to all 6 of the attacks or make a fifth attack with his unarmed strike by accepting only a -2 penalty to all five attacks. He could not however take more than 6 attacks since his level is only 5 meaning he can only take up to 2 additional attacks beyond his normal limit (the feats merely increase his normal limit).

Powergame Analysis: Note that some abilities are meant to be balanced by the fact that low level characters can only perform one attack per round. One example would be a Rogue's Sneak Attack which would greatly increase in power at 1st level by allowing them to hit a flanked target twice. Though normal rules could just as easily allow a Rogue to take Two-Weapon Fighting and do the same thing at a -2 penalty, this does offer a slight advantage at earlier levels while balancing out later. To resolve this, simply state that such abilities function only once per round before 6th level or rule that they function once per round permanently if you believe such abilities to be broken as they are.

This does however add additional attacks at low levels making the risk of death more imminent much like real-world tactics while later levels do not become so needlessly full of repetitive attack rolls designed to miss anyway because of a degrading bonus.